1. Field of the Invention
This invention relates to a magnet roll developing device provided with a non-magnetic rotary sleeve and operative to hold and transfer a magnetic developing agent such as a magnetic two component developing agent composed of a mixture of a magnetic powder carrier and toner, magnetic two component toner developing agent composed of toner containing magnetic powders and frictional charging powders, magnetic one component developing agent composed of toner containing magnetic powders or the like and operative to form a brush fur-shaped portion of the developing agent which acts upon a surface to be developed.
2. Description of the Prior Art
The above mentioned kind of magnet roll developing device has widely been used in an electrophotographic apparatus, electrostatic printer or the like. Such magnet roll developing device, however, has the disadvantage that the electrostatic latent image produced on the surface to be developed, particularly the imagewise dark portion having a large area could not be developed with a sufficiently large concentration and that the developing ability is not always excellent.
In such conventional magnet roll developing device, in order to improve the developing ability, it has been proposed to increase the rotary speed of the non-magnetic rotary sleeve so as to increase the amount of developing agent in contact with unit area of the surface to be developed. But, if the non-magnetic rotary sleeve is rotated at a high speed, much amount of the toner and carrier is scattered. As a result, such conventional developing device has the disadvantage that the developing device and surrounding portions thereof are contaminated, that the non-picture image portion of the surface to be developed becomes overdeveloped, and that the developed picture image becomes uneven in concentration in correspondence with the moving direction of developing agent.
Another conventional developing device in which the developing main magnetic pole is divided into two small magnetic poles which are the same in polarity so as to improve the developing ability has also be proposed. In such conventional developing device, the magnetic lines of force produced from the magnet roll take a configuration which can form two brush fur-like portions thereof. But, if the magnetic developing agent is aligned with the magnetic lines of force, the difference between the density of the magnetic lines of force at the divided two magnetic poles and that in the space between those magnetic poles is not so large that the brush fur-like portions of the developing agent tend to be easily formed in the space. As a result, the brush fur-like portions of the developing agent formed by the two magnetic poles become similar to the brush fur-like portion of the developing agent formed by a single magnetic pole and having a density which is low at the center of the single magnetic pole. Such conventional developing device, therefor, does not contribute to improvement in the developing ability.
In the magnet roll developing device, it is preferable to divide the brush fur-like portion of developing agent at the developing main magnetic pole into a number of brush fur-like portions and to cause these brush fur-shaped portions thus obtained to evenly act upon the surface to be developed for the purpose of developing a picture image having a high revolving power.
But, in the conventional magnet roll developing device, the number of the brush fur-like portions of the developing agent formed by the developing main magnetic pole is minutely changed owing to the distance between a doctor blade and the non-magnetic rotary sleeve, magnetic flux density at the developing main magnetic pole, diameter of the developing agent particles or the like within a limited range. It is impossible to considerably increase the number of brush fur-like portions of the developing agent having a suitable strength. As a result, such conventional developing device has the disadvantage that the picture image having an excellent resolving power can only be developed within a limited range.